Speed of light

[bloodhound31]

A question for those in the know.

I have been reading up on the speed of light to enable me to answer better, the queries I get from the general population.

I am thinking about different wavelengths (Infra-red to Ultraviolet) and our employment of narrow-band filters for astrophotography.

As far as I can make out, light leaves object x, travels through the vacuum at around 300000kmps (all wavelengths the same speed).

When it passes through things like our atmosphere (transparent matter) this has the effect of slowing down the different wavelengths very slightly, breaking them up into the spectrum.

Is this a correct assumption?

How are the colours of the spectrum different from something like, hydrogen-alpha, or oxygen III (filters). Are they just different levels of the same spectrum?

Please keep answers simple for us with no uni degree or mathematical ability.

Baz.

[GeoffW1 (Geoff)]

Hi,

Yes, an ever interesting question.

As far as we know the speed of light is everywhere the same in a vacuum. That is an important proviso. Light certainly does slow down in water, glass, air and a housebrick etc ;=]]

The various colours we see are all different wavelengths of light (but all still the same speed in a vacuum) and these may be separated out when passing at an angle through materials, such as glass.

Newton was not the first to see the spectrum of visible colours, that was Roger Bacon, who saw them in a glass of water (not gin and tonic). In effect he saw his own little rainbow.

Both observed that different wavelengths of light slow down differently in a dense medium. This is not good for us with telescopes, because it means that the colours can all come to a focus at different points, producing the fault called achromatism.

With filters the makers aim to exclude certain wavelengths such as neon or sodium emissions, and allow others through such as HA to enhance our view. These filters are flat plane and not designed to produce any spectrum, only block undesirable colours.

However yes all that is still part of the spectrum.

Cheers

[Robh (Rob)]

Baz,

As you are aware, the colours of light have different wavelengths. Red light has a longer wavelength than blue light, which is more energetic. When light passes from a vacuum into the atmosphere it will slow down marginally but this won't break it up into the spectrum. To break the light up into the colours of the spectrum (dispersion) it must be bent (refracted) through a denser medium. Blue light bends more than red light because of its shorter wavelength. I don't think the atmosphere at higher angles would produce much dispersion. However, water droplets in the air can refract light to produce a rainbow (blue inside the bow, red outside the bow). A glass prism will also refract light, blue bending more than red. The sky appears redder at sunset because other colours are bent more out of our line of sight by particles in the atmosphere.
Through a telescope with poorer optics, you might see colour fringing around stars due to dispersion of light through the glass lenses. As Geoff stated, the different colours are refracted to focus at different points.
A hydrogen-alpha filter passes mainly the red light at the H-alpha wavelength of emission nebulae. A double ionised oxygen filter passes mainly the green light at the O III wavelength of diffuse nebulae and planetary nebulae. As Geoff stated, they are still part of the same light spectrum.

Regards, Rob

[bloodhound31]

Geoff and Rob, thanks so very much for that. It helps when people ask and I need to give them a clear answer without the use of complicated formula and chemistry.

Cheers,

Baz.